Technically this could to some extent be considered Pay to Win. Take for example:
Two different people. One buys RP, the other doesnt. The person buying RP can buy Champions, IP/EXP boost to get them to a higher/max level faster, and get IP at a much quicker rate thus being able to get Runes to better themselves! Whereas the person who didnt have that type of advantage is stuck using a champions they dont like/want to use in order to buy it with IP, and probably waiting longer to get IP to get things like Runes!
But i guess it would depend on what people consider Pay 2 win. Usually when people use that term it means buying something and using it to make you better. usually something like a powerful weapon or armor. Or something that will just level you up, or your stats up without ever doing anything other than buying said item. Whereas with what i used in my example you still need to actually play the game with those boost in order to get better!
Plus there are other circumstances that could prove it false. Said person who bought RP could have bought a champion they thought they would be good with and be crap with it. The could buy IP/EXP boost and never actually play the game during that time.they could buy runes they thought were good, but was bad for said champion! The person who didnt spend a dime could be godllike with the free champions, or cheap champions! all in all you still need to work for things you bought in this game rather than just giving you an advantage just from buying stuff with RP!